1. Field of the Invention
The present invention relates to a fluid swivel and more particularly relates to a multiline piggable fluid swivel which allows a pig used for the removal of deposits from the inner walls of the fluid lines connected to the swivel to readily pass therethrough.
2. Description of the Prior Art
In the marine production and/or transportation of hydrocarbons and related products, it is common to use an offshore terminal to load/offload the hydrocarbons onto or off sea-going tanker vessels. Such offshore terminals are well known and are collectively referred to as single point moors. In carrying out such loading/offloading operations, a vessel is moored to the terminal and flowlines from the vessel are attached to corresponding flowlines on the terminal. Since it is well known that the wind and waves will normally cause a vessel to move (i.e. weathervane) about its mooring point, the flowlines need to be connected through a swivel mounted on the terminal so that the vessel may weathervane during the loading/offloading operations without snarling or damaging the flowlines. Further, since several fluids may be simultaneously loaded/offloaded, the swivel needs to accommodate a multitude of flowlines.
Multiline swivels of the type mentioned above are well known in the art, see, for example, U.S. Pat. Nos. 2,894,268; 3,082,440; 3,351,360; 3,698,433; 4,052,090; 4,111,467 and 4,126,336. However, many of the hydrocarbons (e.g. crude oil) loaded/offloaded through such swivels contain components such as wax or asphalt which can deposit and build up in the flow conduits of the swivel and on the interior walls of the flowlines connected to the swivel. Unless these deposits are routinely removed at desired intervals, flow through the flowlines and swivel may become severely impeded or blocked altogether.
It is common in the petroleum art to remove such deposits from ordinary flowlines by pumping a through-flowline-tool or "pig" through the lines which scrapes or dislodges the deposits from the flowlines so that the deposits are carried out of the lines by the fluids flowing therethrough. However, most multiline swivels of the types disclosed in the above cited patents are not designed to be piggable and will not permit a pig to be pumped therethrough without the risk of sticking the pig in the swivel. When such swivels are used in terminals through which deposit-forming fluids are loaded/offloaded, operations have to be suspended at determined intervals so the flowlines can be pigged and the swivel can be disassembled and cleaned.
To avoid such timely and costly delays, swivels which are piggable, commonly referred to as piggable swivels, have recently been designed and proposed for use in such environments. For example, in U.S. Pat. No. 4,174,127 a piggable swivel is disclosed wherein a guide means is assembled through the outlet conduit of the swivel and projects into the flow path of the swivel to physically catch and direct the pig out of the swivel as it passes therethrough. Further, in U.S. Pat. No. 4,183,559, a piggable swivel is disclosed wherein the fluid path through the swivel is designed so that when the swivel is properly and precisely indexed, the pig can smoothly pass through the swivel.